The Prime Minister could promise to meet his pledge to introduce a married couples' tax allowance in return for support over the issue.

David Cameron addresses guests at a Gay Pride reception in the garden at 10 Downing Street. Photograph: Getty Images.

As the government prepares to announce plans to bring forward legislation on gay marriage, the growing divisions within the Conservative Party over the issue are being exposed. Yesterday, David Davies MP described the policy as "barking mad", adding, apropos of nothing, that "most parents would prefer their children not to be gay". Meanwhile, Peter Bone told Sky News: "It was in no party manifesto, there is no mandate for the Prime Minister to do this; he is absolutely wrong to be doing it now, and he's splitting the Conservative Party when we don't need it to be split."

To counterbalance such figures, a group of senior Conservatives, including Michael Gove, Boris Johnson, Transport Secretary Patrick McLoughlin, and former justice minister Nick Herbert, have founded a new group, Freedom to Marry, to campaign for equal marriage. In a letter to the Sunday Telegraph, they wrote: "We recognise that civil partnerships were an important step forward in giving legal recognition to same sex couples. But civil partnerships are not marriages, which express a particular and universally understood commitment."

Today, they are joined by John Major, who declared in a statement released through the group, "The Prime Minister's instinct to support equal marriage is a courageous and genuine attempt to offer security and comfort to people who - at present - may be together, yet feel apart."

That such senior figures feel the need to campaign for what is, after all, already government policy, is an indication of how weak David Cameron's position is. If the PM is forced to rely on the votes of Labour and Liberal Democrat MPs to pass the bill (at least 130 Tory MPs are prepared to vote against it), he will be exposed as the leader of a divided and, in places, bigoted party. Rather than casting the Conservative Party in a positive light, the issue could only serve as a reminder of how unreconstructed parts of it remain. Cameron will lose Conservative votes over the issue, while failing to gain those of liberals.

If the PM wants to limit the extent of the Conservative rebellion, one option would be to fulfil his long-standing pledge to recognise marriage in the tax system (as ConservativeHome editor Tim Montgomerie has argued). The Coalition Agreement (see p. 30) promised to introduce transferable tax allowances for married couples, while guaranteeing Lib Dem MPs the right to abstain, and the measure is reportedly under consideration for next year's Budget. By pledging to bring forward this policy for straight and gay couples alike, Cameron could drain some of the poison from the Tory revolt.

To be clear, he would be wrong to do so (there is no good argument for privileging married couples over others) and such a move may not even succeed in winning the rebels round. In today's Telegraph, Tory MP Tim Loughton demands the introduction of a tax allowance for married couples, while rebuking Cameron for his support for gay marriage. He writes:

The CSJ’s poll, published today, reveals that not just 47 per cent of Conservative supporters feel betrayed by the PM on this omission [Cameron's failure to introduce a tax allowance for married couples] but 35 per cent of all voters. I doubt that the Government will enjoy anything like compensatory approval ratings for announcing in the same week that gay marriage has apparently become a more urgent issue for Government action, despite no similar manifesto commitment to legislate and after a massive consultation exercise that has been overwhelmingly negative.

All the same, it would be surprising if Cameron wasn't considering a grand bargain along these lines.